Energetics of cubic and monoclinic yttrium oxide polymorphs: Phase transitions, surface enthalpies, and stability at the nanoscale

The energetics of Y2O3 cubic (C-type) and monoclinic (B-type) polymorphs as a function of surface area have been studied by high-temperature oxide melt solution calorimetry and water adsorption calorimetry. The hydrated surface of the monoclinic phase has a lower surface enthalpy (0.64 +/- 0.49 J/m(2)) than that of the cubic (1.25 +/- 0.17 J/m(2)). However, the anhydrous surface of the monoclinic form is less stable (surface enthalpy = 2.78 +/- 0.49 J/m(2)) than that of the cubic (1.66 +/- 0.14 J/m(2)). The phase transformation enthalpy (monoclinic to cubic) for coarse Y2O3 is -21.8 3.2 kJ/mol. An energetic crossover is suggested between the polymorphs under ambient conditions, in which the surfaces are hydrated, at a size of 7 +/- 6 nm; however, anhydrous monoclinic yttria of any particle size is thermodynamically metastable at atmosphere pressure. Surface hydration controls the energetic landscape of Y2O3 polymorphs. The phase transformation process of monoclinic nanocrystals to the cubic phase was also studied using measurements of surface area, X-ray diffraction, and differential scanning calorimetry.